1. Field of the Invention
The present invention relates to a system and method for controlling an internal combustion engine.
2. Description of the Related Art
In recent years, exhaust gas recirculation (EGR) devices that recirculates a portion of exhaust gas to an intake system have been used in internal combustion engines to improve the fuel economy and reduce the amount of nitrogen oxides (NOx) contained in exhaust gas.
If the amount of exhaust gas recirculated by the EGR device (EGR gas) is insufficient, the fuel economy does not improve and NOx cannot be reduced sufficiently. If the amount of EGR gas is excessive, on the other hand, the performance of the engine may be affected. For example, combustion may become unstable, thereby causing misfires.
In general, when internal combustion engines are decelerating, a fuel cut control, in which a throttle valve is fully closed and fuel injection is stopped, is executed to suppress waste of fuel and an increase in the temperature of a catalyst provided in the exhaust system.
When the fuel cut control for deceleration operation is stopped so that normal operation of the engine may resume, the temperature of an air-fuel ratio sensor provided in the exhaust system has decreased and the air-fuel ratio sensor has lost its activity, and therefore feedback control of the air-fuel ratio cannot be restarted immediately. Thus, fuel injection is restarted using a prescribed fuel injection amount until the air-fuel ratio sensor becomes able to function properly. In many cases, the “prescribed amount” is determined according to the amount of intake air detected by an air flow meter when the fuel cut control is stopped.
Now, the effect of EGR gas at the time when the fuel cut control is stopped to resume normal operation is considered. If EGR is performed by the EGR device described above when the fuel cut control is started, some EGR gas may not have been scavenged but remains in the intake system even when the fuel cut control is stopped. In this way, the amount of fresh air that is actually inducted into a cylinder is less than the amount of air detected by the air flow meter when the fuel cut control is stopped. As a result, the air-fuel ratio may become excessively rich, which may cause unstable combustion such as misfires.
The limit of the amount of EGR gas that may be inducted into the cylinder increases with increasing load on or increasing speed of the internal combustion engine. That is, when the fuel cut control is stopped to resume normal operation and when the internal combustion engine is operating under a low load and at a low speed, the limit of the amount of EGR gas that may be inducted into the cylinder is low. Thus, if the amount of EGR gas remaining in the intake system when the fuel cut control is stopped exceeds the limit, combustion may become unstable.
One of the techniques related to EGR gas remaining in intake systems is made to solve the problem that EGR gas remaining in an exhaust system of a hybrid vehicle (HV) is not purified by a catalyst when fuel is cut during deceleration. In this technique, a clutch between a motor and an engine is engaged for motoring and a throttle valve is fully opened during deceleration to feed the remaining EGR gas to the catalyst for purification (see Japanese Patent Application Publication No. 2002-256919 (JP-A-2002-256919)).
Although the remaining EGR gas can be scavenged efficiently, this technique is not applicable to vehicles other than HVs. In addition, the deceleration feel is diminished because the throttle valve is fully opened during deceleration, which is another disadvantage.
Other proposed related arts include a technique to suppress a decrease in amount of intake air to be charged, due to EGR gas remaining in an intercooler, by causing the intake air to bypass the intercooler when operation of an internal combustion engine is shifted from an EGR induction region to a non-EGR induction region (see Japanese Patent Application Publication No. 6-257518 (JP-A-6-257518)).